1. Field of the Invention
The present invention generally relates to systems and methods for inspecting a wafer with increased sensitivity. Certain embodiments relate to systems and methods for inspecting a specimen that include replacing a gas proximate to an illuminated spot on the wafer with a medium that scatters less light than the gas thereby increasing the sensitivity of the inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various times during a semiconductor manufacturing process to detect defects on wafers. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
As the semiconductor industry gears up for the 45 nm node and beyond, integrated circuit (IC) manufacturers are demanding better sensitivity for unpatterned wafer laser surface scanning inspection systems. For example, some IC makers are requiring 25 nm polystyrene latex (PSL) sphere sensitivity on bare polished silicon wafers for three generations beginning at the 45 nm node. This sensitivity is also required at a throughput of 45 wafers per hour. The best performance of most commercially available inspection systems does not meet these sensitivity and throughput performance requirements.
Often, increased sensitivity can be achieved by system configurations that result in reduced throughput. For example, the sensitivity of currently available inspection systems can be increased by reducing the size of the spot on the wafer that is illuminated during inspection. The size of the illuminated spot on the wafer may be reduced relatively simply in many currently used inspection systems (e.g., by altering or adding an optical element to the beam forming optics train). Reducing the spot size effectively decreases the amount of light that is scattered from the surface of the wafer relative to the defect scattering, thereby increasing the defect signal to noise ratio and the sensitivity of the system. However, reducing the spot size also reduces the throughput of the system since scanning a smaller size spot over an entire wafer surface takes longer than scanning a larger size spot over the wafer surface. Therefore, by varying the spot size, it is possible to trade-off throughput for sensitivity.
Other changes can also or alternatively be made to currently available inspection systems to increase the sensitivity of the inspection systems. For example, the collector of some currently available inspection systems may be altered by changing or adding an aperture to the collector. The aperture may be configured to block light that is scattered from the surface of the wafer while allowing light scattered from a defect to pass through the aperture thereby increasing the defect signal to noise ratio of the system and the sensitivity of the system. In another example, the light source of currently available inspection systems may be replaced with a higher power light source. For example, if an inspection system is configured for a laser power of about 350 mW, the laser power of the system can be increased to about 1000 mW. Increasing the power of the light source generally increases the level of light scattered from defects thereby increasing the sensitivity of the system.
Improvements in the surface quality of wafers may also effectively increase the sensitivity of currently used inspection systems. In particular, as the residual surface roughness of wafers such as bare silicon wafers decreases, the amount of light scattered from the wafer surface (i.e., “background scattering”) will also decrease. Therefore, the defect signal to noise ratio will increase thereby increasing the sensitivity of the system.
The various improvements in inspection systems and wafer surfaces described above can be used in combination to increase the sensitivity of the systems to various degrees. For example, adding a back aperture as described above and improving the wafer surface may produce a larger increase in sensitivity than adding the back aperture alone. In addition, the degree to which this combination of improvements increases the sensitivity increases with greater improvements in the wafer surface (e.g., from a background scatter of about 30 ppb of the incident laser power to about 15 ppb of the incident laser power). Furthermore, adding a back aperture, improving the wafer surface, and increasing the laser power produces a larger increase in sensitivity than that which can be achieved by adding the back aperture and/or improving the wafer surface.
Obviously, each of the different improvements described above may be implemented at different costs. For instance, improving the sensitivity of the systems by utilizing an aperture and relying on improvements in wafer surfaces is less expensive than increasing the laser power. However, an increase in the laser power may be required to meet the sensitivity requirements described above. Nevertheless, improving sensitivity as described above relies on improvements in wafer surface quality from the typical background scattering observed today (about 80 ppb of the incident laser power) beyond the level of today's best wafer surfaces (background scattering of about 30 ppb of the incident laser power) to ultra-smooth silicon with background scattering of about 15 ppb of the incident laser power.
At such low levels of wafer-induced background scattering or haze, laser scanning technology has reached the point at which Rayleigh scattering from the air that the laser beam passes through contributes a significant component of the overall background scattering observed. For instance, for some currently available inspection systems, the estimated background scattering due to atmospheric Rayleigh scattering is about 10 ppb of the incident laser power to about 20 ppb of the incident laser power. Obviously, however, the background scattering is dependent on the parameters of the inspection system configuration such as wavelength of illumination, polarization of illumination, optical path length, solid angle of the collector, the depth of field, collection polarization; etc. Therefore, due to atmospheric Rayleigh scattering, a wafer with actual 15 ppb background scattering would look to the detection system like a wafer that has background scattering of about 25 ppb of the incident laser power to about 35 ppb of the incident laser power. Therefore, improvements in sensitivity expected based on improvements in the wafer surface quality cannot be achieved.
Accordingly, it would be advantageous to increase the sensitivity of systems and methods for inspecting a wafer by reducing scattering of light caused by a gas proximate to an illuminated spot on the wafer.